1. Field of the Invention
The present invention relates to a rail-to-rail operational amplifier and, more particularly, to a rail-to-rail operational amplifier with an enhanced slew rate.
2. Description of the Related Art
FIG. 1 is a circuit diagram showing a conventional rail-to-rail operational amplifier 10. Referring to FIG. 1, the rail-to-rail operational amplifier 10 has a complementary differential input stage, which is constructed by a first and a second P-type transistors PQ1 and PQ2 and a first and a second N-type transistors NQ1 and NQ2. A source electrode of the first P-type transistor PQ1 and a source electrode of the second P-type transistor PQ2 are coupled together. A drain electrode of the first P-type transistor PQ1 and a drain electrode of the second P-type transistor PQ2 are coupled to a summing output stage 11, respectively. A constant high-side bias current source ICH is coupled between a high-side supply voltage VH and the mutually-coupled source electrodes of the first and the second P-type transistors PQ1 and PQ2. A first input voltage Vinp is applied to a gate electrode of the first P-type transistor PQ1 while a second input voltage Vinn is applied to a gate electrode of the second P-type transistor PQ2. A differential voltage DV may be defined as the first input voltage Vinp minus the second input voltage Vinn, i.e., DV=(Vinp−Vinn). Under the control of the differential voltage DV, the constant high-side bias current source ICH is divided to flow through the first and the second P-type transistors PQ1 and PQ2
A source electrode of the first N-type transistor NQ1 and a source electrode of the second N-type transistor NQ2 are coupled together. A drain electrode of the first N-type transistor NQ1 and a drain electrode of the second N-type transistor NQ2 are coupled to the summing output stage 11, respectively. A constant low-side bias current source ICL is coupled between the mutually-coupled source electrodes of the first and the second N-type transistors NQ1 and NQ2 and a low-side supply voltage VL. The first input voltage Vinp is applied to a gate electrode of the first N-type transistor NQ1 while the second input voltage Vinn is applied to a gate electrode of the second N-type transistor NQ2. Under the control of the differential voltage DV, the constant low-side bias current source ICL is divided to flow through the first and the second N-type transistors NQ1 and NQ2.
The summing output stage 11 combines the four current signals from the complementary differential input stage, i.e. the four current signals from the transistors PQ1, PQ2, NQ1, and NQ2. On a basis of such combination, the summing output stage 11 generates an output voltage Vout.
From the point of view with respect to a common mode voltage VCM, the operation of the rail-to-rail operational amplifier 10 may be divided into three ranges. Within a low range of VL<VCM<(VL+Vtn), wherein Vtn is the turn-on threshold voltage of the N-type transistor, the operation of the rail-to-rail operational amplifier 10 is executed only from the high-side differential input pair constructed by the first and the second P-type transistors PQ1 and PQ2 because the first and the second N-type transistors NQ1 and NQ2 are disabled. Within a high range of (VH−|Vtp|)<VCM<VH, wherein Vtp is the turn-on threshold voltage of the P-type transistor, the operation of the rail-to-rail operational amplifier 10 is executed only from the low-side differential input pair constructed by the first and the second N-type transistors NQ1 and NQ2 because the first and the second P-type transistors PQ1 and PQ2 are disabled. Within an intermediate range of (VL+Vtn)<VCM<(VH−|Vtp|), the operation of the rail-to-rail operational amplifier 10 is executed from both of the high-side and the low-side differential input pairs because the first and the second P-type transistors PQ1 and PQ2 and the first and the second N-type transistors NQ1 and NQ2 are all enabled.
The rail-to-rail operational amplifier 10 has an advantage of allowing the common mode voltage VCM to be applicable within the whole range from VL to VH. In light of the development trend that the power supply voltage of today's electronic devices is continuously reduced, such advantage makes possible the whole range of the power supply voltage to be effectively used by the rail-to-rail operational amplifier 10.
On the other hand, today's electronic devices are required to operate at a higher speed of transmitting electronic data. More specifically, when the input voltage Vinp and/or Vinn of the operational amplifier changes, the output voltage Vout of the operational amplifier will responsively change from an original state into another state. A rate of change of the output voltage with respect to time is often referred to as the slew rate. The higher the slew rate, the faster the speed of the operational amplifier. However, the conventional rail-to-rail operational amplifier 10 as shown in FIG. 1 fails to provide a higher slew rate than other operational amplifiers of different types.
Therefore, it is desirable to provide a rail-to-rail operational amplifier with an enhanced slew rate.